1. Field of the Invention
The present invention relates to an image forming apparatus periodically varying a difference in moving speed between an image bearing member and a moving member in a transfer nip for transferring a toner image onto the image bearing member.
2. Description of the Related Art
In an image forming apparatus in utilization of an electrophotographic system, an electrostatic transfer system is generally used as a transfer unit. That is, with a transfer corotron, a transfer bias roller and the like, transfer medium (a member to become an object where an image is transferred) is provided with charge in polarity opposite from the polarity of toner to, thereby, move a toner image developed onto an image bearing member onto transfer medium with an electrostatic force. However, in such an image forming apparatus, only the power of electric field is occasionally low in transfer efficiency of toner on the image bearing member onto a transfer medium. In addition, defects in transfer such as transfer missing and the like occasionally take place. Therefore, further improvement in transfer efficiency is called for.
Japanese Patent Application Laid-Open No. H09-197846 proposes prevention of transfer defects by varying speed of one of a transfer bias roller and a middle transferring belt. Thereby, by providing toner with oscillation to one of the sheet conveyance direction and the middle transferring member conveyance direction at the occasion of transfer, decrease in adhesion of toner to a photosensitive drum is intended. In the invention described in Japanese Patent Application Laid-Open No. H09-197846, one of transfer bias roller and a drive roller of a middle transferring member is driven with a stepping motor to provide oscillation in utilization of speed fluctuation inevitably taking place in principle on a step basis at an occasion when the stepping motor is driven.
However, in the invention described in Japanese Patent Application Laid-Open No. H09-197846, a speed fluctuation in a transfer bias roller and a drive roller of a middle transferring member is originated from the step angle of a stepping motor and, therefore, it is impossible to provide a fluctuation with an angle smaller than the minimum step angle. Normally, the step angle of the stepping motor is set based on drive torque realized by a motor, the rotational speed at the time of use and the like and is inferior in degree of freedom in setting of the step angle of the stepping motor. That is, actually, it is difficult to design a stepping motor providing a desired speed oscillation. Moreover, in the invention described in Japanese Patent Application Laid-Open No. H09-197846, the speed oscillation is intrinsic to a stepping motor and cannot be changed. The speed difference cannot be reset appropriately in conformity with use status.
On the other hand, U.S. Pat. No. 5,329,341 has proposed a system including a photosensitive belt being an image bearing member provided with oscillation with an ultrasonic resonator from the rear side of a toner retaining surface to increase toner delamination on a photosensitive belt to promote transfer. The invention described in U.S. Pat. No. 5,329,341 is adapted to include an oscillation unit of an ultrasonic resonator brought into a state in contact to a photosensitive belt in a configuration provided with walls in its periphery so that the interior enclosed by the walls is decompressed to retain tight contact and oscillation from the oscillation unit is transferred to the photosensitive belt. Moreover, the resonator is adapted to be driven at an average frequency of the primary and the secondary resonance frequency in a portion of the photosensitive belt spanned and retained by the walls in an intention that a fluctuation of oscillation amplitude hardly takes place for a fluctuation such as an impact due to sheet feeding.
However, the invention described in U.S. Pat. No. 5,329,341 requires an oscillation source called resonator besides the drive source to move the photosensitive belt, complicating the apparatus.
The present invention is to utilize a motor being a drive source of an image bearing member and a moving member to provide an appropriate speed fluctuation.